1. Field of the Invention
The present invention relates generally to low compression multi-piece golf balls having a solid core and multi-layered cover, wherein at least one of the cover layers is made of a plasticized thermoplastic composition. Preferably, the ball has a compression of less than 60 and more preferably less than 50. In one example, a two-layer cover including inner and outer cover layers may be made. In another example, there is a three-layer cover including inner, intermediate, and outer cover layers. Preferably, the plasticized thermoplastic composition comprises an ethylene acid copolymer ionomer and plasticizer.
2. Brief Review of the Related Art
Multi-layered, solid golf balls are used today by recreational and professional golfers. In general, these golf balls contain an inner core protected by a cover. The core acts as the primary engine for the ball and the cover protects the core and helps provide the ball with durability and wear-resistance. The core and cover may be single or multi-layered. For example, three-piece golf balls having an inner core, inner cover layer, and outer cover layer are popular. In other instances, golfers will use a four-piece ball containing a dual-core (inner core and surrounding outer-core layer) and dual-cover (inner cover layer and surrounding outer cover layer). Intermediate layer(s) may be disposed between the core and cover layers to impart various properties. Thus, five-piece and even six-piece balls can be made. Normally, the core layers are made of a natural or synthetic rubber material or an ionomer polymer. These ionomer polymers are typically copolymers of ethylene and methacrylic acid or acrylic acid that are partially or fully neutralized. In particular, highly neutralized polymer (HNP) compositions may be used to form a core layer. Metal ions such as sodium, lithium, zinc, and magnesium are commonly used to neutralize the acid groups in the copolymer.
Such ethylene acid copolymer ionomer resins generally have good durability and toughness. These ionomers may be used to make cover, intermediate, and core layers for the golf ball. When used as a core material, the ionomer resin helps impart a higher initial velocity to the golf ball. When used as a cover material, the ionomer resin helps impart impact durability, wear-resistance, and cut/shear-resistance to the golf ball.
Many golf balls used today have multi-layered covers comprising an inner cover and surrounding outer cover layer. For example, the inner cover may be made of a relatively hard material such as the above-described ethylene acid copolymer ionomer resin. Meanwhile, the outer cover layer may be made of a relatively soft polyurethane or polyurethane composition. Also, three-layered covers, wherein the cover structure includes an inner cover, intermediate cover, and outer cover layer may be made. For example, Sullivan, US Patent Application Publication 2002/0151380 discloses a golf ball having a core and cover wherein the cover comprises: a) an inner cover layer having a first thickness and being disposed directly about the core; b) an outer cover layer having a second thickness no greater than about 0.050 inches; and c) an intermediate cover layer having a third thickness and being disposed between the inner and outer cover layers; wherein the outer cover layer comprises a composition formed of a reactive liquid material (thermoset material comprising polyurethane, polyurea, polyurethane ionomer, epoxy, or a mixture thereof) and the combination of the first, second, and third thickness is no greater than about 0.125 inches.
Sullivan et al., US Patent Application Publication 2004/0235587 discloses a golf ball having a core and a cover comprising: an inner cover layer; an outer cover layer having a material hardness of 60 Shore D or less; and an intermediate cover layer disposed between the inner and outer cover layers. At least two of the inner, intermediate, and outer cover layers comprise a non-ionomeric material. Preferably, the outer cover layer comprises a polyurethane, a polyurea, a copolymer of a polyurethane, a copolymer of a polyurea, or an interpenetrating polymer network.
In recent years, golf balls having a lower compression and spin rate have been developed. For example, three-piece balls containing an inner core and outer core layer (or intermediate layer) made of polybutadiene rubber and outer cover made of ionomer resin have been developed. These relatively low compression and low spin balls are particularly desirable when making driver shots off the tee, because the balls generally have good flight distance. Also, lower compression balls have a generally softer feel and many golfers find that a softer, more natural feel is advantageous when making putt shots on the greens of a golf course. The feel of the ball primarily depends upon the hardness and compression of the ball.
Golf balls having a relatively low compression are described in the patent literature. For example, Ogg et al., U.S. Pat. No. 7,918,748 discloses a two-piece golf ball comprising a core comprising made of a neodymium-catalyzed polybutadiene composition, and a cover made of partially-neutralized ethylene acid copolymer composition, wherein the golf ball has a diameter of at least 1.68 inches, a mass of at least 45 grams, a coefficient of restitution of at least 0.780, and a PGA compression ranging from 35 to 50. Lemons et al., U.S. Pat. No. 7,431,669 discloses a low compression golf ball, wherein the core is formed of polybutadiene rubber that produces a zero (or less) compression. The mantle is molded around the core using ethylene, acrylic acid and n-butyl acrylate terpolymers, with 100% of the acrylic acid groups neutralized with metal ions. The cover is molded using conventional ethylene acid copolymer ionomers. Cavallaro, U.S. Pat. No. 5,947,842 discloses a low-spin multi-layer golf ball containing a core, a cover layer, and a mantle layer disposed between the core and cover layer. The core preferably has a compression of less than 80 points and more preferably less than 60 points. According to the '842 Patent, calcium oxide can be added to the core composition in place of zinc oxide and this helps reduce core compression by at least about 2 compression points. Loper et al., US Patent Application Publication 2009/0170634 discloses a golf ball comprising: (a) a core; (b) an inner mantle layer; (c) an intermediate mantle layer; (d) an outer mantle layer; and (e) at least one cover layer; wherein the mantle layers may be made of various ionomer compositions, and the core has a PGA compression of less than 70, and the combined sub-assembly of core/inner mantle layer/intermediate mantle layer has a PGA compression of at least 30.
Although some of the above-described compositions may be somewhat effective for making certain components and layers in a multi-piece golf ball, there is still a need for new compositions that can impart high performance properties to the ball. In particular, there is a continuing need for improved cover constructions in golf balls. The cover material should help provide the ball with a relatively low compression and a soft feel. The ball should have good spin control. At the same time, the impact durability and resiliency (as measured by Coefficient of Restitution) properties of the ball should not be sacrificed. The ball should have high impact durability and resiliency. The present invention provides golf balls having an optimum combination of properties.